1. Field of the Invention
The present invention relates to a gas sensor and, more particularly, to a gas sensor provided with a protector for covering a portion of a bottomed cylindrical or plate-shaped gas detecting element that is exposed to a gas to be measured.
2. Description of the Related Art
When the air/fuel ratio of an automobile is to be controlled, a gas sensor provided with a gas detecting element having electric characteristics varying with the concentration of a specific gas component in the exhaust gas is conventionally used. A variety of sensors such as an oxygen sensor, a NOx sensor or an HC sensor are known. In this gas sensor, the gas detecting element is held in a metal shell such that its leading end portion protrudes therefrom. The specific gas component (e.g., oxygen) in the gas to be measured such as the exhaust gas is detected by exposing the leading end portion of the gas detecting element to the measured gas.
In most gas sensors, moreover, a metallic protector for covering the leading end portion is mounted on the metal shell so that the leading end portion of the gas detecting element to be exposed to the measured gas is protected from contact with water. This protector has a plurality of gas communication holes, through which the measured gas is introduced into the protector and contacts the leading end portion of the gas detecting element. In order to enhance protection of the leading end portion of the gas detecting element, moreover, the protector most frequently used in recent years has employed a dual structure composed of two members, namely an inner cover portion and an outer cover portion. A gas sensor having such a protector is disclosed in JP-A-9-304332, JP-A-2000-28571 and JP-A-2001-99807, for example.
3. Problems to be Solved by the Invention
Various methods have been adopted for fixing the protector and the metal shell. In order to fix these two components reliably, a method has also been frequently adopted in which a cylindrical protector is fitted on the radially outer side of the leading end portion of the metal shell and in which a welded portion (e.g., a laser welded portion) is formed for the fitting portion. In case the protector is to be fitted on the leading end portion of the metal shell, a lubricant is applied to the surface of the metal shell. As such, the fitting between the inner circumference of the protector and the outer circumference of the leading end portion of the metal shell may be smoothed by reducing friction therebetween, to thereby fit the protector by means of the lubricating action of the lubricant. In mass production of the sensor, the lubricant is ordinarily applied to the whole surface of the metal shell. When the protector is to be fitted on the leading end portion of the metal shell, press-fitting ordinarily is also adopted so as to ensure a fitting relation.
If, however, the fitting portion, in which the protector is fitted by applying the lubricant to the surface of the metal shell, is welded (or lap-welded), the gas of the lubricant component resulting from heat input of the welding operation has no clear avenue of release, thereby causing a welding defect such as a blow hole. It is, therefore, conceivable to selectively remove, after applying lubricant to the surface of the metal shell, that lubricant on the portion of the metal shell which is to form the welded portion. However, such selective removal requires much step management, thus raising problems of lowered manufacturing efficiency and increased cost.